Not applicable.
This invention relates to ultrasonic diagnostic systems which measure the velocity of fluid flow using spectral Doppler techniques. In particular, the invention relates to automatic adjustment of velocity scale and pulse repetition frequency for such systems.
Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. In the measurement of blood flow characteristics, returning ultrasonic waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers, such as blood cells. This frequency shift translates into the velocity of the blood flow.
In state-of-the-art ultrasonic scanners, the pulsed or continuous wave (CW) Doppler waveform is computed and displayed by a video processor in real-time as a gray-scale spectrogram of velocity versus time with the gray-scale intensity (or color) modulated by the spectral power. The data for each spectral line comprises a multiplicity of frequency data bins for different frequency intervals, the spectral power data in each bin for a respective spectral line being displayed in a respective pixel of a respective column of pixels on the display monitor. Each spectral line represents an instantaneous measurement of blood flow.
Each vertical line in the spectral display corresponds to a Doppler frequency spectrum at a given time instant. Positive Doppler frequencies correspond to flow towards the transducer, and negative frequencies correspond to flow away from transducer. The Doppler shift limits are +/xe2x88x92PRF/2, where PRF is the pulse repetition frequency, or the sampling frequency of the Doppler signal. Given the Doppler beam to vessel angle (which is specified by the user via a Doppler angle cursor graphic over the B-mode image), the Doppler frequency shifts can be converted into velocity units according to the standard Doppler equation.
In practice, over the FFT analysis time interval, the velocity distribution within the pulsed Doppler sample volume in a vessel can be positive or negative depending on the flow direction with respect to the transducer. If the Doppler spectrogram display is inverted because the flow is all negative, the user can press an invert key to invert the Doppler spectral lines, such that xe2x88x92PRF/2 maps to the top of the spectrogram display.
Aliasing occurs when the pulse repetition frequency (PRF) is below the Nyquist rate of the random Doppler signal, and for example results in the positive frequency spectrum being wrapped around to the negative frequency axis. Provided that the total spectral bandwidth is less than PRF, a simple shift of the baseline, fshift can effectively unwrap the spectrum. In conventional Doppler scanners, such baseline shifts are usually effected by the operator via a toggle switch on the front panel. Often this will require the user to toggle the baseline shift key a few times depending on the fshift step size.
If the spectrum bandwidth is larger than PRF, however, it can not be unwrapped simply by adjusting the polarity or baseline position of the velocity scale. In that case, the user must increase the PRF thereby extending the limits of the velocity scale via a front-panel control switch. On the other hand, if the spectrum bandwidth is much smaller than the current PRF setting, the user will usually reduce the PRF to expand the height of the spectrogram in the display area.
Typical clinical Doppler exams can be time-consuming since they involve adjustment of a variety of control keys and switches for sample volume size, flow direction cursor angle, velocity limits (PRF), baseline shift and invert, auto max/mean velocity trace etc. As a result, there is a need for automating some of these basic Doppler adjustments in order to improve both the speed and reliability of the Doppler exam.
The preferred embodiment of the invention is useful in an ultrasound system comprising a Doppler image display unit for displaying a Doppler image with an adjustable polarity and an adjustable position along the frequency axis. The embodiment improves the appearance of the Doppler image by transmitting ultrasound waves at a predetermined pulse repetition frequency, preferably by an ultrasound transmitter unit. Doppler signals are generated in response to the ultrasound waves backscattered from a subject under study, preferably by an ultrasound receiver unit. Memory signals having memory values are stored. The memory signals include first signals having first values representing the magnitudes of at least some of the component frequencies of the Doppler signals generated over a predetermined time period in response to the backscattered ultrasound waves due to fluid flow in a first direction and second signals having second values representing the magnitudes of at least some of the component frequencies of the Doppler signals generated over the predetermined time period in response to the backscattered ultrasound waves due to fluid flow in a second direction opposite the first direction. The storing preferably is accomplished in a digital memory. A third signal having a third value related to a noise signal level present in the system is generated. The memory values are analyzed based at least in part on the third value, preferably by a logic unit. One or more of the polarity, position and pulse repetition frequency (PRF) may be adjusted in response to the analyzing whereby the Doppler image generated by the Doppler image display unit is changed.
By using the foregoing methods and apparatus, the polarity and position of a Doppler image are automatically adjusted and the PRF is automatically adjusted, thereby aiding the reading and interpretation of the Doppler image. For example, if the spectrum is aliased, it will be automatically unwrapped via baseline shift and/or an increase in PRF. If the spectrum is inverted, it can be automatically inverted back for an upright display. If the spectrum bandwidth is too small relative the current PRF or velocity limits, the PRF is automatically reduced for optimal display. These automated velocity scale and PRF adjustments improve the ease-of-use of the Doppler equipment, and ultimately the speed and reliability of the Doppler exam.